94 



SPONGES 



type the sponge has more the form of a growth, spreading or compact, 

 without distinct individuals. In the Leucosolenia type the sponge appears 

 as a collection of distinct Olynthus individuals, each throwing out diver- 

 ticula on every side, from which daughter individuals arise by a process 

 of budding. In Clathrina the intercanal system is greatly developed ; in 

 Leucosolenia the term can scarcely with justice be applied to the inter- 

 spaces between the diverticula and oscular tubes. 



In the family Clathrinidae the canal system, though always reducible 

 to the type above described, may undergo certain secondary modifications 

 which may be considered under two heads, according as they affect the 

 gastral cavity or the intercanal system. As an instance of the former 

 kind may be mentioned the frequent widening of the cavity of the 



osc.t 



FlO. 65. 



Types of canal system in Ascons. The thick black line represents the gastral layer, the 

 dotted line the dermal layer; the pores are not represented. A, Clathrina type; B, Leuco- 

 solenia type, osc.t, oscular tube ; t.s, tubar system ; i.c, intercanal system. 



central oscular tube, until it assumes the appearance of a central cloaca 

 or basin, into which the Ascon tubes empty themselves. This modifica- 

 tion has reached its limit in the species Clathrina tripodifera, Carter (type 

 of Bidder's genus Dendya), as described by Dendy (1891), in which the 

 tubar system takes on a radiate arrangement round the very large central 

 cloaca. In the genus Ascandra, on the other hand, the gastral cavity is 

 divided up by folds of the gastral layer, which owe their origin to the 

 great development of the spicule rays which project from the wall intc 

 the gastral cavity. The diverticula thus formed are not, however, in any 

 way comparable to those seen in the oscular tube of Leucosolenia, since in 

 Ascandra the folding does not affect the external surface of the body wall, 

 but only the gastral layer. 



Modifications of the intercanal system in the simple Clathrina type 



